Nadal-Ferret Marc, Gelabert Ricard, Moreno Miquel, Lluch José M
Departament de Química, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain.
Phys Chem Chem Phys. 2015 Dec 14;17(46):30876-88. doi: 10.1039/c5cp01067e.
In this paper, we have analyzed the feasibility of spontaneous proton transfer in GFP at the Franck-Condon region directly after photoexcitation. Computation of a sizeable portion of the potential energy surface at the Franck-Condon region of A the structure shows the process of proton transfer to be unfavorable by 3 kcal mol(-1) in S1 if no further structural relaxation is permitted. The ground vibrational state is found to lie above the potential energy barrier of the proton transfer in both S0 and S1. Expectation values of the geometry reveal that the proton shared between the chromophore and W22, and the proton shared between Ser205 and Glu222 are very close to the center of the respective hydrogen bonds, giving support to the claim that the first transient intermediate detected after photoexcitation (I0*) has characteristics similar to those of a low-barrier hydrogen bond [Di Donato et al., Phys. Chem. Chem. Phys., 2012, 13, 16295]. A quantum dynamical calculation of the evolution in the excited state shows an even larger probability of finding those two protons close to the center compared to in the ground state, but no formation of the proton-transferred product is observed. A QM/MM photoactive state geometry optimization, initiated using a configuration obtained by taking the A minimum and moving the protons to the product side, yields a minimum energy structure with the protons transferred and in which the His148 residue is substantially closer to the now anionic chromophore. These results indicate that: (1) proton transfer is not possible if structural relaxation of the surroundings of the chromophore is prevented; (2) protons H1 and H3 especially are found very close to the point halfway between the donor and acceptor after photoexcitation when the zero-point energy is considered; (3) a geometrical parameter exists (the His148-Cro distance) under which the structure with the protons transferred is not a minimum, and that, if included, should lead to the fluorescing I* structure. The existence of an oscillating stationary state between the reactants and products of the triple proton transfer reaction can explain the dual emission reported for the I0* intermediate of wtGFP.
在本文中，我们分析了光激发后直接在弗兰克 - 康登区域绿色荧光蛋白（GFP）中自发质子转移的可行性。对该结构在弗兰克 - 康登区域相当一部分势能面的计算表明，如果不允许进一步的结构弛豫，在S1态中质子转移过程是不利的，能量上不利3千卡/摩尔。发现基态振动状态在S0和S1态中均高于质子转移的势能垒。几何结构的期望值表明，发色团与W22之间共享的质子以及Ser205与Glu222之间共享的质子非常接近各自氢键的中心，这支持了光激发后检测到的第一个瞬态中间体（I0*）具有类似于低势垒氢键的特征这一说法[迪多纳托等人，《物理化学化学物理》，2012年，13卷，16295页]。激发态演化的量子动力学计算表明，与基态相比，在激发态中发现这两个质子靠近中心的概率甚至更大，但未观察到质子转移产物的形成。使用通过取A最小值并将质子移至产物侧获得的构型启动的QM/MM光活性态几何优化，得到了质子转移后的最低能量结构，其中His148残基与现在带阴离子的发色团实质上更接近。这些结果表明：（1）如果阻止发色团周围环境的结构弛豫，质子转移是不可能的；（2）考虑零点能时，光激发后质子H1和H3尤其非常接近供体和受体之间的中点；（3）存在一个几何参数（His148 - Cro距离），在该参数下质子转移后的结构不是最小值，如果考虑该参数，应该会导致荧光I结构。三重质子转移反应的反应物和产物之间存在振荡稳态可以解释野生型绿色荧光蛋白I0中间体报道的双发射现象。
